WDM PON uses different wavelengths for each ONU which increases bandwidth per user or ONU when compared with other PON technologies using shared wavelengths such as GPON. Other WDM PON advantages include broadband user connections, symmetric downstream/upstream traffic, service transparency, possibility of stepwise system upgrade, long distance, channels segregation, and low latency. Because a dedicated wavelength is used for each ONU the data processing requirements are simplified at the OLT or central office and the ONU. This is because the individual users do not need to be individually addressed as they would in a shared wavelength PON, because the dedicated wavelength is only intended for one ONU. The need for addressing requires layer 2 and layer 3 processing equipment at the OLT and ONU which increases their complexity and cost. Thus for example traffic for a particular ONU must be packaged to be carried in a dedicated timeslot of the shared channel, or addressed using Ethernet packets for example. By comparison, in a WDM PON the traffic can be processed in the OLT at layer 1 and the incoming traffic directly converted to optical signals for transmission to the ONU, rather than requiring packing into addressed packets for example. This layer 1 processing considerably simplifies the central office equipment and reduces cost.
Whilst point-to-point traffic has been predominant in PON traffic to date, increasingly PON is also carrying multi-cast and broadcast traffic. Point-to-point traffic occurs when a user or ONU communicates with one other terminal point, for example a website or video-on-demand (VOD) server. Increasingly multicast/broadcast traffic such as television channels are being provided in which traffic from one source is transmitted to a number of end users such as ONU. In this situation the traffic is in one direction only (downstream) and is provided (logically) in parallel to each end user. In practice this can be achieved in PON using layer 2 processing in which the broadcast channels are packaged into Ethernet frames or into a dedicated downstream timeslot which all ONU can access.
Multicast or broadcast services in a WDM PON can be carried at Layer2 or Layer3, by allocating a portion the downstream frames sent to the final users. Alternatively these multicast services can be packaged into Layer2 packets and added to each or the respective ONU wavelength. However, this prevents the aforementioned advantages of only using Layer1 processing at the OLT from being utilized. Layer1 processing could be maintained by utilizing tunable filters at the ONU to receive broadcast or point-to-point channels on different wavelengths as required. However the use of tunable filters at ONU is expensive and the need for power splitting of the common broadcast channels reduces their power and hence robustness and range or distance. In a further alternative parallel fibers could be employed but this is very expensive.